The present invention is directed to a method and apparatus for simultaneously perforating casing to permit production from two or more production zones. More particularly, the present invention comprises a collapsible spacer for properly positioning each of at least two perforating guns within casing adjacent one of a plurality of production zones, which spacer can substantially reduce its profile when allowed to drop into the bottom of the wellbore below the production zones (known as the "rathole") after the guns have been fired and the tool released.
In drilling oil and gas wells, multizone production is frequently encountered, i.e., more than one reservoir will be located with a single wellbore. Once the full series of casing strings have been set, one of two perforating techniques have been used in the past to permit communication between the production zones and the casing interior.
First, the lowermost zone could be perforated, the well killed by pumping mud downhole to balance the pressure of the production fluids, and then the lower zone packed off to allow a second (and subsequent) perforating gun(s) to be run in to complete one or more additional zones above the first. This method has several disadvantages. It requires multiple trips into the hole which adds to the time and expense of completing the well. In addition, a production zone may be fluid sensitive in which case the mud used to kill the well could contaminate the formation. Further, killing the lower zone not only adds expense, but risks plugging some or all of the newly formed perforations in both the casing and the formation. In a sense, such a procedure has steps which offset or fight one another, a first step to encourage fluids to flow, a subsequent step to curtail (at least temporarily) the flow of those fluids.
A second technique involves the use of a plurality of perforating guns positioned in the desired relationship by an intervening spacer. This technique has the advantage of requiring only a single tool run in and, if the expended tool can be discarded into the rathole, the well need not be exposed to fluid used to kill it, as would be necessary if the tool were retrieved. A disadvantage associated with dropping the expended tool in the rathole is that the borehole has to be drilled a sufficient additional depth below the lowermost production zone to accommodate the full length of the tool or else the upper portion of the tool will interfere with the free flow of production fluids into the casing. As the spacing between the multiple producing zones increases, this can add significant additional time and cost to the drilling of the borehole.
The method and apparatus of the present invention has all the advantages of the second technique without the drawbacks. The spacer used to position the multiple perforating guns, is itself a multicomponent element. A first segment can receive at least one additional segment telescopically, permitting the spacer to collapse to the length of 1/2 (or less) of the gun spacing. A set of pins maintains the desired spacing between the guns under influence of a movable piston. A longitudinal bore through the piston affords a passageway for the Primacord connecting the plural perforating guns. Upon firing of the perforating guns, the piston is caused to slide out of engagement with the pins which then disengage the two segments permitting their collapse when the tool is dropped into the rathole.
Various other characteristics, features, and advantages of the present invention will become apparent after a reading of the following detailed description.